Wednesday September 30 2009
Reducing excess body fat is associated with good health
“Middle-aged spread can knock years off your life,” the Daily Mail has warned, saying that piling on weight in middle age can cut your chances of living to an old age by 80%.
This news is based on a well-conducted study that had followed 121,700 nurses since 1976, carrying out regular and comprehensive assessments of them. It does suggest, as may be expected, that if you are overweight, reduced weight and body fat is associated with good health and survival. The research team also adjusted to account for various social, demographic and lifestyle factors that may influence the association between weight and health.
It should be noted that the study did not assess the chance of survival, but of ‘healthy survival’ at age 70 or over (the researchers’ own measure of disease-free living), which was seen in only 9.9% of participants. While there are some limitations to the study, its conclusions agree with traditional advice that the best way to live a healthy life is to eat a balanced diet, exercise regularly and avoid habits that put your health at risk, such as smoking and excess alcohol.
Where did the story come from?
The story was published in the British Medical Journal and authored by Qi Sun and colleagues of Harvard School of Public Health and the University of Warwick. The study was funded by the National Institutes of Health in the US, and the Pilot and Feasibility Program sponsored by the Boston Obesity Nutrition Research Center.
What kind of scientific study was this?
This was a cohort study designed to examine the theory that mid-life “adiposity” (a build-up of body tissue that stores fat) is linked to a reduced chance of maintaining optimal health in older age.
The study involved participants of the Nurses’ Health Study, which started in 1976 and enrolled 121,700 healthy women aged between 30 and 55. Questionnaires were sent to the participants upon entry into the study (baseline) and at various points during the follow-up period, asking questions on disease, lifestyle and medical risk factors. Food frequency questionnaires were used in 1980 and repeated every two to four years.
A 36-item health status survey was included on the 1992, 1996, and 2000 questionnaires, and included questions on physical activity and perception of health. Major chronic diseases (including coronary disease, cancer and diabetes) were self-reported and then confirmed through medical records. From 1995 onwards, 93% of nurses aged 70 or older had their cognitive function (thought processes including reasoning, thinking and perception) assessed.
Weight and height were collected during the baseline questionnaire, with further weight assessments every two years thereafter. Although weight was self-reported, a validity study was carried out in 184 women and showed a strong correlation between self-reported and measured weights.
In a 1986 assessment of central obesity, the participants' BMI was calculated and measures of waist circumference, hip circumference, and waist-to-hip ratio were taken. The study population for this analysis was, however, only a subset of the primary study population (9,512 for waist circumference; 9,450 for hip circumference; 9,438 for waist-hip ratio). When conducting analyses between these measures and survival, researchers adjusted for various social and demographic factors that might affect the relationship.
The final follow-up for the purposes of this study was in 2000, when 95% of the cohort could be contacted. Deaths were identified by reports from next of kin, postal authorities or through the national death index. Almost all those who had died could be identified. The researchers defined healthy survivors as women who survived to age 70 or older and were at that point free from 11 major chronic diseases, had no major limitations of physical function, had intact cognitive function and were in good mental health.
What were the results of the study?
Only 1,686 (9.9%) of the surviving cohort met the criteria for a “healthy survivor”. A total of 15,379 (90.1%) were “usual survivors”, who had various impairments of either chronic disease, cognitive function or mental health, physical health, or a combination of these. In comparison, healthy survivors were typically healthier at baseline, and were more likely to have a better diet and education. They were also less likely to have overall or central obesity in 1986, to have gained less weight since age 18, and to smoke.
After adjusting for various lifestyle and dietary variables, there was a significant trend of a reduced chance of healthy survival in those women with a higher BMI at baseline or greater waist circumference, hip circumference or hip-to-waist ratio in 1986. Compared with women of BMI 18.5 to 22.9, obese women (with a BMI of more than 30) had a 79% decreased risk of healthy survival (odds ratio 0.21, 95% confidence interval 0.15 to 0.29).
The chance of healthy survival after age 70 was also decreased with weight gained from age 18 until middle age. For women who were both overweight (a BMI of more than 25) and had gained at least 10kg (22lbs) since age 18, there was an 82% decreased risk of being a healthy survivor compared to women of lean BMI and who had remained at a relatively stable weight.
What interpretations did the researchers draw from these results?
The researchers say that their findings give evidence that adiposity in middle-aged women is strongly related to a reduced risk of healthy survival into older age.
What does the NHS Knowledge Service make of this study?
This is a very well-conducted study that has followed a large group of women over a considerable period of time using regular and comprehensive assessments. It does suggest, as one might expect, that reduced weight and adiposity in overweight people is associated with healthy survival. In their analyses they also adjusted for various social, demographic and lifestyle factors that may affect the association. However, there are still related issues to consider:
- Although the news highlights that middle-aged spread reduces your chance of surviving by 80%, it is not survival that is being assessed: it is healthy survival. This measure was designed by the study authors to mean the absence of any of 11 major chronic diseases, physical limitations, cognitive impairment or mental health issues at age 70. Very few people in this cohort met these full criteria, and it may be expected that a comparatively low proportion of the general population aged 70 or over would be able to meet all of them. Outside this study context, there is no validated definition of 'healthy survivor' in general use.
- Most women were under 75 at the study’s end, and so survival rates into older ages cannot be accurately determined.
- The adiposity measures were conducted at one time point in 1986, and the women may have changed over time.
- Weight and measurements were self-reported, which may introduce inaccuracies (although the researchers did make efforts to account for this through their validation assessment).
- Although some news reports feature photographs of men, this was a study of women only. Results from this study may not be generalised as applying to men. Additionally, all participants were nurses, a specific social group that may not be comparable to all other populations. The women were also primarily of white ethnicity.
The researchers say that their study “emphasises the importance of maintaining a healthy weight from early adulthood”. Despite the limitations, their conclusions agree with traditional advice that, although all genetic and medical predispositions to disease may not be controllable, the best way to live a healthy life is to eat a balanced diet, exercise regularly and to avoid unhealthy habits such as smoking and excess alcohol.